1. Field of the Invention
The present invention relates to a power supply circuit for a gate driving circuit for driving power semiconductor switching devices of a power converter such as an inverter, in particular to such a circuit in a power conversion circuit that uses flying capacitors.
2. Description of the Related Art
FIG. 8 is a circuit diagram of the main circuit of a two-level inverter circuit, a representative power conversion circuit, performing DC to AC power conversion. The inverter circuit of FIG. 8 includes a main AC power source APM, a rectifying circuit RE composed of diodes and other circuit elements for converting an AC power to a DC power, and a DC intermediate circuit corresponding to a DC power supply generally composed of large capacity capacitors Ca and Cb. When a rectified DC voltage is higher than the rated voltage of the capacitor, the two capacitors are connected in series as shown in FIG. 8. The inverter system of FIG. 8 includes a load such as an AC motor ACM and a DC to AC conversion circuit INV having power semiconductor devices, the conversion circuit INV delivering variable voltages at variable frequencies. When power is regenerated from the load, the inverter main circuit operates as a converter to convert the AC power to DC power.
The DC to AC conversion circuit INV includes semiconductor switching devices Su, Sv, Sw, Sx, Sy, and Sz each having an IGBT (insulated-gate bipolar transistor) and an antiparallel-connected diode. A three-phase-output circuit includes six circuits of the semiconductor switching devices. The power converter further includes gate driving circuits GDu, GDv, GDw, GDx, GDy, and GDz for driving the IGBTs, and a control circuit CNT for controlling the power converter circuit. The control circuit CNT gives an ON-OFF command signal, a gate driving signal, to a gate driving circuit for each IGBT. Because the base potential of the control circuit generally differs from the potential at the IGBTs and potential at their gate driving circuit, power supply to the gate driving circuit needs an insulating device such as a transformer.
FIGS. 9A and 9B show examples of circuits for supplying power to a gate driving circuit GD from a low voltage AC power supply AP at a commercial frequency. The low voltage AC power of the power supply AP is usually supplied by the main AC power source APM indicated in FIG. 8. The system of FIG. 9A generates high frequency AC from the low voltage AC power supply AP through an AC/AC conversion circuit ACV or an AC/DC/AC conversion circuit, insulates the high frequency AC through an insulating device of a high frequency transformer HFT, converts the insulated high frequency AC into DC power with a diode D and a capacitor Cd, and supplies the DC power to a gate driving circuit GD for an IGBT S. Here, the high frequency AC helps to miniaturize the transformer HFT. FIG. 9B shows a system using an insulating device of a commercial frequency insulating transformer CFT. This system eliminates the AC/AC conversion circuit ACV in the system of FIG. 9A and establishes electrical insulation maintaining at the commercial frequency. In this construction, however, the insulating transformer CFT operates at a commercial frequency, and thus has a larger size than the transformer HFT in the system of FIG. 9A.
For the high frequency transformer HFT and the commercial frequency transformer CFT for use in the apparatus for driving motors of 200 V system and 400 V system, a withstand voltage around 2 kV is sufficient. However, for a power supply to a gate driving circuit for IGBTs used in a high voltage apparatus of several kilo-volts, a transformer with a withstand voltage over 10 kV is needed.
FIG. 10 shows a high voltage power conversion circuit based on the circuit of FIG. 8. This circuit example, disclosed in Japanese Unexamined Patent Application Publication No. 2009-177951, is a flying capacitor type power conversion circuit. This circuit does not use a high withstand voltage semiconductor switching device, but uses low withstand voltage semiconductor switching devices connected in series. The circuit further includes a flying capacitor connected in parallel to the series circuit of semiconductor switching devices. Although FIG. 10 shows a circuit for three-phase AC output, only the U-phase is described below, because the three phases have the same circuit construction. The U-phase includes a series circuit of four semiconductor switching devices Su1, Su2, Sx1, and Sx2 between a positive terminal P and a negative terminal N of a DC power supply consisting of DC single power supplies DP1, DP2, DN1 and DN2 connected in series. A series circuit of flying capacitors Cu1 and Cu2 is connected between the connection point between the switching devices Su1 and Su2 and the connection point between the semiconductor switching devices Sx1 and Sx2. In the case that the voltage of the DC power supply is 4Ed and the potential at the point M, which is a middle potential point of the DC power supply, is defined to be the base potential zero, three levels of potentials 2Ed, 0, and −2sEd can be delivered at an AC output point A by controlling the flying capacitor voltage to be 2×Ed. Thus, the circuit of FIG. 10 is a three-level inverter.
FIGS. 11 and 12 show constructions of gate driving power supplies, wherein the circuit of FIG. 11 includes one transformer for each IGBT whereas the circuit of FIG. 12, which is disclosed in Japanese Unexamined Patent Application Publication No. 2006-081232, includes two series-connected transformers for each IGBT. The high frequency transformer HFT1 shown in FIG. 11 is provided for the purpose of electrical insulation between the low voltage AC power supply AP and the main circuit, and the high frequency transformer HFT3 shown in FIG. 12 is provided for the purpose of electrical insulation between the low voltage AC power supply AP and the potential at the point M of the DC power supply. Both the transformers HFT1 and HFT3 need usually to have high withstand voltage. The transformers HFT2 shown in FIG. 12 for supplying power to gate driving circuits for IGBTs operates with reference to the base potential at the point M and need a withstand voltage of 2×Ed.
High voltage apparatuses of several kilovolts as described above generally use high withstand voltage transformers to electrically insulate the power supplies for gate driving circuits for every IGBT. This causes a high cost. Transformers insuring a high withstand voltage must be ensured a sufficiently large insulation distance between the primary side and the secondary side, which causes the transformers to be large-sized. The cost and volume of the transformer are not simply proportional to the magnitude of the withstand voltage but increase in an exponential manner. Thus, the reduction of cost and volume is a severe challenge in high voltage apparatuses.
The three-level inverter shown in FIG. 11 and higher levels of multilevel inverters generally contain a large number of semiconductor switching devices, which in turn requires correspondingly many transformers of high withstand voltage, thereby causing a rise of cost. In the circuit having the structure of FIG. 12, the primary winding potential of the high frequency transformers HFT2 is the potential at the point M, but the transformers HFT2 need to insure a withstand voltage of at least 2 Ed, which is a half of the main circuit DC power supply voltage.